Blender 2.77 Motion Vectors
Some times vector passes like depth or motion can't be displayed directly because they are 'out of range' of displayable values (I think , not really sure)

Anyway all you need to do is to bring these values within range by using a Vector > Normalize node

Have in mind that to actually use the vector pass the original 'non displayable' values are sometimes the correct ones that will produce the desirable outcome even if they can't be displayed correctly directly.